Electrophilic aromatic substitution. Part XVI. Detritiation and desilylation of 1,6-methanoannulene and 11,11-difluoro-1,6-methanoannulene
The first quantitative determination of the electrophilic aromatic reactivity of an annulene has been made through measurements of rates of protiodetritiation and protiodesilylation. For 11,11-difluoro-1,6-methanoannulene partial rate factors for the 2-and 3-positions in detriation are 3 750 and 161, respectively, leading to corresponding σ+ values of –0.41 and –0.25; the partial rate factor for desilylation of the former position is 20.2. For 1,6-methanoannulene the partial rate factors for reaction at the 2-position were 10 300 000 and 9 270 in detritiation and desilylation, respectively, leading to a σ+ value of –0.80; the lower stability of this compound to acids prevented measurement of the reactivity of the 3-position. The normality of both these compounds toward selectrophilic substitution is shown by the fact that the reactivities of the 2-positions of each compound in both reactions are closely similar to those obtained with some conventional aromatic compounds viz. the 2-positions of dibenzofuran and thiophen, respectively.